Chemical Industry
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Transcript of Chemical Industry
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Chemical Industry Chemical reactions and physical
processes on a large scale to convert raw materials into useful products.
Conditions of the reactions are controlled to produce the best yield of product possible at an economic rate.
YIELD: Quantity of product formed Theoretical: Predicted by equation Actual: Quantity actually obtained 1
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Haber Process Process to produce ammonia. Developed by German Chemist Fritz
Haber N2(g) + 3H2(g) 2NH3(g) H = –46kJmol-1CONDITIONS FOR HIGH YIELD High pressure (less molecules on the
product side) Low temperature (forward reaction is
exothermic)
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Haber Process
ACTUAL CONDITIONS High Pressure (200-250
atmospheres pressure). If pressure is too high, expensive structural requirements are needed for the plant.
Moderately high temperature (~ 400oC). If the temperature is low then the yield is high, but it takes a long time for the reaction to produce the product (Rate low) 3
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Haber Process Iron catalyst
increases rate of forward and back reaction.
Yield of ammonia is approximately 45% of the theoretical yield
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Haber Process
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Contact Process Product of sulfur dioxide from
sulfur or metal sulfides
S(s) + O2(g) SO2(g) 2ZnS(s) + O2(g) 2ZnO(s) +SO2(g)
Conversion of sulfur dioxide to sulfur trioxide
2SO2(g) + O2(g) 2SO3(g) H = –99kJmol-1
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Contact Process Absorption of sulfur trioxide into
concentrated sulfuric acid to form oleum
SO3(g) + H2SO4(l) H2S2O7(l)
REACTION THAT CONTROLS YIELD
2SO2(g) + O2(g) 2SO3(g) H = –99kJmol-1
CONDITIONS FOR HIGH YIELD High pressure Low temperature
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Contact ProcessACTUAL CONDITIONS Atmospheric pressure. Yield is about
85-90% at this pressure. Costs to increase pressure are not offset by much greater yield.
Temperature: 450oC. Compromise between yield and rate.
Vanadium pentoxide catalyst (V2O5) increases rate of reaction.
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Contact Process
Sulfur trioxide is dissolved in concentrated sulfuric acid as it forms to maximise yield.
Acid is transported as oleum (less corrosive) and diluted as required by buyer which reduces transport costs.
H2S2O7(l) + H2O(l) 2H2SO4(aq)
600kJ of energy is released for every mole of acid formed. Some of this energy is used to produce electricity for the plant.
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Flow Diagrams Used to represent the movement
of materials through various components of the plant.
May include diagrams of equipment or show the process through a series of boxes and arrows. May show quantities of material and energy.
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Haber Process
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Contact Process
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Flow Diagrams
RAW MATERIALS Converted by chemical/ physical
means into useful products. Examples include coal, oil, natural gas, air, limestone, sand, metal ores, water
WASTE PRODUCTS No use or market for the product.
Disposal can be a problem if they are toxic or produced in large amounts.
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Flow Diagrams
BY PRODUCTS Not the main product, but do have
a use either within the plant or commercially. E.g. sulfur dioxide from metal smelters
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Metals
Most occur in the earth’s crust as minerals
The most common occurrences are K, Ca, Na, Mg as salts(Cl–, SO4
2–, CO3
2–) Al, Fe, Sn as oxides Zn, Ni, Pb, Cu as sulfides Au, Ag, Pt as the uncombined
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Metal Reactivity When metals react they undergo
oxidation (lose electrons) M Mx+ + xe
More easily a metal is oxidised, the less easily its ions are reduced to the metal
When determining the reactivity of a metal its reactions with water, acid and metal displacement reactions are considered
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Metal Reactivity Example: Reactions of Calcium Water: Ca(s) +H2O(l) Ca(OH)2(s)
+ H2(g)
Acid: Ca(s) + 2H+(aq) Ca2+
(aq) + H2(g)
Displacement: Ca(s) + Zn2+
(aq) Ca2+
(aq) + Zn(s) 17
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Metals from their Ores
Ore deposit is a region in the earth’s crust where the concentration of a metallic mineral is at a level where the extraction of the metal is commercially viable
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Metals from their Ores
Concentration of the mineral (removal of the gangue)
Conversion of the concentrate into a substance suitable for reduction. (Most common chemical process metal sulfide to metal oxide)
Reduction of the metal compound to metal via chemical means or electrolysis.
Refining the metal to remove trace impurities 19
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Production of Zinc
Zinc ore (zinc blende) is mined at Broken Hill (NSW) and Mt Isa (Qld). Contains approx 2-8% zinc
Crushed and ground into small particles at the mine ready for froth flotation
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Froth Flotation
Ore is added to tanks containing water, frothing agents and collector molecules (molecules with polar and non polar ends)
ZnS is attracted to the polar end of collector molecules and is carried to the surface of tanks on the froth when air is blown through the mixture. This is skimmed off.
Gangue remains on the bottom of the tank as a sludge 21
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Froth Flotation
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Production of Zinc The zinc sulfide is roasted in air to
form zinc oxide
2ZnS(s) + 3O2(g) 2ZnO(s) + 2SO2(g)
Sulfur dioxide is used to make sulfuric acid for next step (Contact process)
Oxide is leached with sulfuric acid
ZnO(s) + H2SO4(aq) ZnSO4(aq) + H2O(l)
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Production of Zinc
Zinc powder is added to displace less reactive metals (Ag+, Cd2+, Cu2+). These are collected and processed.
Electrolysis of zinc sulfate Anode (Lead or silver/lead)
2H2O(l) O2(g) + 4H+
(aq) + 4e Cathode (aluminium or zinc)
Zn2+(aq) +2e Zn(s)
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Production of Zinc
Overall 2Zn2+
(aq) +2H2O(l) 2Zn(s) +O2(g) + 4H+
(aq) The zinc produced is 99.95% pure
and requires no further purification
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Reduction using Electrolysis
Metals more active than zinc can’t be produced by electrolysis of aqueous solutions.
If a solution of a more active metal is electrolysed then
2H2O(l) + 2e H2(g) + 2OH–(aq)
occurs at the cathode in preference to the reduction of the metal.
A molten electrolyte is required with metals above zinc
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Reduction of Aluminium
Molten alumina Al2O3 is mixed with cryolite Na3AlF6, CaF2 and AlF3 .
This mixture has a melting point of ~1000oC compared to alumina which melts at 2030oC
This means the electrolysis is carried out at a lower temperature saving money
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Reduction of Aluminium
Anode (Carbon) 2O2–
(l) O2(g) +4e
C(s) + O2(g) CO2(g) The anode is eaten away and
requires regular replacement.Cathode (Carbon lined steel
tank) Al3+
(l) + 3e Al(l) The aluminium forms below the
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Reduction of Aluminium
Overall 4Al3+
(l) +6O2–(l) +3C(s) 4Al(l) + 3CO2(g)
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Chemical Reduction
Metals below aluminium can be produced by reduction with carbon
Iron: 3C(s) + Fe2O3(s) 2Fe(s) + 3CO(g)
Zinc: C(s) + ZnO(s) Zn(s) + CO(g) These metal are more easily
reduced than metals higher in the reactivity series
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Energy
The Reduction stage consumes most energy and so is the most costly stage of any metal production.
Electrolysis of a molten (non aqueous) electrolyte requires the more energy than other methods of reduction.
Consequently it is preferable (cost wise) if a metal can be either chemically reduced or produced by electrolysis of an aqueous solution.
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Energy
Chemical Reduction
Electrolysis of aqueous solution
Electrolysis of molten liquid
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Most Energy required: Most expensive
Least Energy required: Least expensive